Localization Of Valence Electrons Research Articles

The effects of V doping on the intrinsic properties of SmFe10Co2 alloys: A theoretical investigation

The present study focuses on the intrinsic properties of the SmFe10Co2-xVx (x = 0–2) alloys, which includes the SmFe10Co2 alloy, one of the most promising permanent magnets with the ThMn12 type of structure due to its large saturation magnetization (µ0Ms = 1.78 T), high Curie temperature (Tc = 859 K), and anisotropy field (µ0Ha = 12 T) experimentally obtained. Unfortunately, its low coercivity (<0.4 T) hinders its use in permanent magnet applications. The effect of V-doping on magnetization, magnetocrystalline anisotropy energy, and Curie temperature is investigated by electronic band structure calculations. The spin-polarized fully relativistic Korringa-Kohn-Rostoker (SPR-KKR) band structure method, which employs the coherent potential approximation (CPA) to deal with substitutional disorder, has been used. The Hubbard-U correction to local spin density approximation (LSDA + U) was used to account for the large correlation effects due to the 4f electronic states of Sm. The computed magnetic moments and magnetocrystalline anisotropy energies were compared with existing experimental data to validate the theoretical approach’s reliability. The exchange-coupling parameters from the Heisenberg model were used for obtaining the mean-field estimated Curie temperature. The magnetic anisotropy energy was separated into contributions from transition metals and Sm, and its relationships with the local environment, interatomic distances, and valence electron delocalization were analyzed. The suitability of the hypothetical SmFe10CoV alloy for permanent magnet manufacture was assessed using the calculated anisotropy field, magnetic hardness, and intrinsic magnetic properties.

Temperature-dependent work function, thermionic emission and bulk modulus of TiC: a study on the identification of free valence electrons and localized valence electrons and their roles played in carbides.

By analyzing the projected states of valence electrons (fatband structures), the localized valence electrons and the free valence electrons of TiC were identified, respectively. After defining the volumes and the magnitudes of localized valence electrons and free valence electrons, the influences of the temperature, including the thermal expansion and the atomic thermal vibration, on the localized valence electron density and the free valence electron density were investigated, respectively. Based on the metallic plasma model (MPM), the temperature-dependent work functions and the thermionic emission current densities of TiC were calculated in terms of temperature-dependent free valence electron densities. The results were in good agreement with experimental results. Furthermore, as it was observed, the linear dependence of the bulk modulus on the localized valence electron density demonstrated that the bulk modulus of TiC was determined by the localized valence electron density. The different roles played by the free valence electrons and the localized valence electrons in the work function and the bulk modulus of TiC could be attributed to their different contributions to the kinetic energy density of valence electrons. The influences of the temperature on the work function, thermionic emission and bulk modulus of TiC indicated that the transition metal carbides with lower free valence electron density, higher localized valence electron density and heavier atomic mass were desired to achieve lower work function, higher current density and higher stability.

Superstructures and Chemical Bonding in Rare Earth Metal Polytellurides RETe2‐δ (RE=La−Nd; Sm−Tm; 0≤δ≤0.2)

AbstractPolychalcogenides REX2‐δ (X=S, Se, Te; 0≤δ≤0.2) of trivalent rare earth metals RE have been investigated in recent years to shed light on the structural diversity as a function of compositional, metric, thermodynamic, and electronic situation. Whereas the former aspects have comparable influence on the structures of all polychalcogenides REX2‐δ, the bonding situation was assumed different for tellurides due to tellurium's higher tendency to delocalize electrons. The crystal structures generally contain puckered [REX] double slabs and planar [X] layers, the latter hosting different distortions from a square‐like arrangement. The distortion patterns of sulfides and selenides can be understood by a Zintl‐type approach; they are dominated by localization of valence electrons in mono‐ (X2−) or dinuclear (X22−) anions only. This review discusses crystal structures of some rare‐earth metal polytellurides RETe2‐δ (0≤δ≤0.2) and bonding features in the chalcogenide layers and relates them to their sulfide and selenide counterparts.

Dirac nodal-line semimetal zinc polynitride at high pressure

Topological semimetals exhibit novel quantum states and electronic structures with interesting Fermi surfaces. In contrast to Weyl and Dirac nodal points, nodal lines can have multiple topological configurations in momentum space that lead to new properties, such as long-range Coulomb interaction and flat Landau levels. Herein, we report the discovery of a Dirac nodal-line semimetal polynitride ${\mathrm{ZnN}}_{4}$, synthesized from the elements under high pressure via laser-heated diamond anvil cell technique. The crystal structure of ${\mathrm{ZnN}}_{4}$ has an $Ibam$ space group, and features infinite one-dimensional nitrogen chains in the pressure range of 14--130 GPa. The Dirac cone is observed midway between $S$ and $X$ points in the Brillouin zone and the Dirac nodal-line is found near the $T$ point. The measured electrical resistance of the ${\mathrm{ZnN}}_{4}$ sample shows an anomalous increase with pressure, which is attributed to the localization of valence electrons. The case of ${\mathrm{ZnN}}_{4}$ provides a unique example of Dirac nodal-line semimetal in nitrides.

Simple hexagonal structured gold with eight-coordination formed with ordered structural vacancies

Phase engineering of nanomaterials attracts increasing research interest, as crystal structures of metals often determine their physical and chemical properties. Metals tend to form close-packed crystal structures to maximize their space filling and atomic coordination numbers due to the isotropic metallic bonding. Hitherto no metals yet crystallize with the simple hexagonal packing which has low coordination number and packing fraction. Here, we fabricate a novel simple hexagonal structured gold through in situ selective etching on ordered Cu-Au intermetallic compound, where the new crystal structure has only 8 nearest neighbors for each atom and a low packing fraction of 0.60. The simple hexagonal structured Au demonstrates an electrical conductivity of 389 S/m, approximately 105 times lower than that of face-centered cubic structured gold, according to first principles calculation, associated with localized valence electrons. The discovery of simple hexagonal structure suggests that metals can form more diverse crystal structures by changing effective atomic volume, which provides a pathway to engineer new structures of metals for novel physical properties and applications.

First principles design of 2 dimensional Nickel dichalcogenide Janus materials NiXY (X,Y = S,Se,Te)

In this work, we propose novel two-dimensional (2D) Janus Ni dichalcogenide materials and explore their feasibility, stability and evaluate their electronic and optical properties with ab-initio calculations. Three unique Janus materials, namely NiSSe, NiSTe and NiSeTe, with 2H hexagonal structure were proposed. Density functional theory (DFT) calculations, show that among the three proposed NiXY Janus 2D materials, NiSSe had the best energetic and dynamical stability. GGA PBE calculations showed NiSSe to have a metallic bandstructure with the Ni-Se interaction having a dominant role in the band profile near the Fermi energy. Electron localization function (ELF) and total potential plots show a distinguishable asymmetry in terms of valence electron localization and distribution between the S and Se atoms in 2D NiSSe. The presence of large amount of electron gas like feature in the ELF around the chalcogen atoms also indicates their importance in the conduction properties. Optical properties calculated with random phase approximation (RPA) show the 2D NiSSe to have broad spectrum optical response with significant peaks lying in each of the infra-red, visible and the ultraviolet range of the spectra.

A new explanation on valence electron structure of C, Si, and Ge crystals with diamond structure based on photoelectron spectra

According to the classical covalent-bond (CVB) model, the two s and two p valence electrons in C, Si, and Ge crystals with diamond structure adopt an equivalent sp3 structure and form four covalent bonds between an atom and its four adjacent atoms. Therefore, the four valence electrons (sp3) per atom should have the same energy state. However, many experimental results indicate that the valence electrons of these element crystals have different binding energies. Therefore, in this paper, we propose a local valence electron (LVE) model for these materials, on the basis of valence electron spectra for the C, Si, and Ge crystals: The states of the four valence electrons per atom in these crystals are similar to those in the free atoms (s2p2), except that the shape of their outer electron cloud shells deviates from the ball shell distribution in the crystals, forming positive ions and equivalent-negative- electrical-charge centers. One of the important reasons why all the valence electrons of diamond are local electrons is that free C atom has high first ionization energy, 11.26 eV, being much higher than those of all the metal atoms. This LVE model is accordance with that diamond has wide energy gap (5.4 eV) in the energy band theory, which forbids electron transition from the filled valence band to the vacant conduction band.

Anion-exchange-mediated internal electric field for boosting photogenerated carrier separation and utilization

Heterojunctions modulated internal electric field (IEF) usually result in suboptimal efficiencies in carrier separation and utilization because of the narrow IEF distribution and long migration paths of photocarriers. In this work, we report distinctive bismuth oxyhydroxide compound nanorods (denoted as BOH NRs) featuring surface-exposed open channels and a simple chemical composition; by simply modifying the bulk anion layers to overcome the limitations of heterojunctions, the bulk IEF could be readily modulated. Benefiting from the unique crystal structure and the localization of valence electrons, the bulk IEF intensity increases with the atomic number of introduced halide anions. Therefore, A low exchange ratio (~10%) with halide anions (I–, Br–, Cl–) gives rise to a prominent elevation in carrier separation efficiency and better photocatalytic performance for benzylamine coupling oxidation. Here, our work offers new insights into the design and optimization of semiconductor photocatalysts.

Identifying Metallic Transition-Metal Dichalcogenides for Hydrogen Evolution through Multilevel High-Throughput Calculations and Machine Learning.

High-performance electrocatalysts not only exhibit high catalytic activity but also have sufficient thermodynamic stability and electronic conductivity. Although metallic 1T-phase MoS2 and WS2 have been successfully identified to have high activity for hydrogen evolution reaction, designing more extensive metallic transition-metal dichalcogenides (TMDs) faces a large challenge because of the lack of a full understanding of electronic and composition attributes related to catalytic activity. In this work, we carried out systematic high-throughput calculation screening for all possible existing two-dimensional TMD (2D-TMD) materials to obtain high-performance hydrogen evolution reaction (HER) electrocatalysts by using a few important criteria, such as zero band gap, highest thermodynamic stability among available phases, low vacancy formation energy, and approximately zero hydrogen adsorption energy. A series of materials-perfect monolayer VS2 and NiS2, transition-metal ion vacancy (TM-vacancy) ZrTe2 and PdTe2, chalcogenide ion vacancy (X-vacancy) MnS2, CrSe2, TiTe2, and VSe2-have been identified to have catalytic activity comparable with that of Pt(111). More importantly, electronic structural analysis indicates active electrons induced by defects are mostly delocalized in the nearest-neighbor and next-nearest neighbor range, rather than a single-atom active site. Combined with the machine learning method, the HER-catalytic activity of metallic phase 2D-TMD materials can be described quantitatively with local electronegativity (0.195·LEf + 0.205·LEs) and valence electron number (Vtmx), where the descriptor is ΔGH* = 0.093 - (0.195·LEf + 0.205·LEs) - 0.15·Vtmx.

Hubbard-corrected density functional perturbation theory with ultrasoft pseudopotentials

We present in full detail a newly developed formalism enabling density functional perturbation theory (DFPT) calculations from a $\mathrm{DFT}+U$ ground state. The implementation includes ultrasoft pseudopotentials and is valid for both insulating and metallic systems. It aims at fully exploiting the versatility of DFPT combined with the low-cost $\mathrm{DFT}+U$ functional. This allows us to avoid computationally intensive frozen-phonon calculations when $\mathrm{DFT}+U$ is used to eliminate the residual electronic self-interaction from approximate functionals and to capture the localization of valence electrons, e.g., on $d$ or $f$ states. In this way, the effects of electronic localization (possibly due to correlations) are consistently taken into account in the calculation of specific phonon modes, Born effective charges, dielectric tensors, and in quantities requiring well converged sums over many phonon frequencies, as phonon density of states and free energies. The new computational tool is applied to two representative systems, namely CoO, a prototypical transition metal monoxide and ${\mathrm{LiCoO}}_{2}$, a material employed for the cathode of Li-ion batteries. The results show the effectiveness of our formalism to capture in a quantitatively reliable way the vibrational properties of systems with localized valence electrons.

Fisher information for endohedrally confined hydrogen atom

Abstract Electron localization and delocalization of endohedrally confined hydrogen atom has been investigated employing Fisher information theory. Confinement has been modeled using a spherical Gaussian-type potential. B-spline bases expansion method was used to solve the Schrodinger equation to obtain the required energy eigenvalues and the corresponding wave functions. Changes in energies with depths of potential are explained using Hellmann-Feynman theorem. The behavior of Fisher information against the confining potential depths and positions are demonstrated. Moreover, our results show that Fisher information is an effective way to measure the localization of valence electrons.

(Invited) Ultrafast X-Ray Studies of Interfacial Energy- and Charge-Transfer Dynamics

The success of many emerging molecular electronics concepts hinges on an atomistic understanding of the underlying electronic dynamics. Processes evolving on spatial and temporal scales spanning orders of magnitude have to be connected in order to gain a comprehensive understanding of the fundamental dynamics and scaling laws that enable molecular, interfacial, and macroscopic charge and energy transport. Time-domain X-ray spectroscopy techniques have the potential to provide a deeper understanding of electronic dynamics in complex, heterogeneous systems owing to their elemental site specificity and sensitivity to local valence electron configurations. We present femtosecond to picosecond time-resolved X-ray photoelectron spectroscopy (TRXPS) studies of photoinduced charge transfer dynamics in nanoporous films of N3 dye-sensitized ZnO and bilayer heterojunctions consisting of copper phthalocyanine (CuPc) electron donors and C60 acceptors. For the N3/ZnO system, differential TRXPS line shifts provide site-specific access to the locations of intermittently trapped electrons, interfacial dipoles, as well as charge delocalization and recombination dynamics. For the CuPc/C60 heterojunction, a deeper understanding of the predominant energy transport and charge generation mechanisms is achieved. Contrary to common belief, fast intersystem crossing from initially excited singlet excitons to triplet excitons within the bulk of the donor domain is not a loss channel but the triplet excitons contribute to a significantly larger extent to the time-integrated interfacial charge generation than the initially excited interfacial singlet excitons. The TRXPS data provide direct access to the diffusivity of the triplet excitons in the CuPc donor domain DCuPc = (1.8 ±1.2) × 10−5 cm2/s and their diffusion length Ldiff = (8 ± 3) nm. Preliminary TRXPS results will be presented for a model system for plasmon-enabled photocatalytic nano-assemblies consisting of a nanoporous TiO2 layer sensitized with gold nanoparticles (Au NPs). Clear differences between the temporal responses of the Au4f and Ti2p photolines are observed, indicating both the interfacial site-specificity of photoinduced electronic dynamics and the capacity of TRXPS to distinguish them. Figure 1

Energetics and cathode voltages of LiMPO4 olivines ( M=Fe , Mn) from extended Hubbard functionals

Transition-metal compounds pose serious challenges to first-principles calculations based on density-functional theory (DFT), due to the inability of most approximate exchange-correlation functionals to capture the localization of valence electrons on their $d$ states, essential for a predictive modeling of their properties. In this work we focus on two representatives of a well known family of cathode materials for Li-ion batteries, namely the orthorhombic LiMPO$_4$ olivines (M = Fe, Mn). We show that extended Hubbard functionals with on-site ($U$) and inter-site ($V$) interactions (so called DFT+U+V) can predict the electronic structure of the mixed-valence phases, the formation energy of the materials with intermediate Li contents, and the overall average voltage of the battery with remarkable accuracy. We find, in particular, that the inclusion of inter-site interactions in the corrective Hamiltonian improves considerably the prediction of thermodynamic quantities when electronic localization occurs in the presence of significant interatomic hybridization (as is the case for the Mn compound), and that the self-consistent evaluation of the effective interaction parameters as material- and ground-state-dependent quantities allows the prediction of energy differences between different phases and concentrations.

From LaAlO3/SrTiO3 to LaAlO3/KNbO3: Improving the transport properties of two-dimensional electronic gas in created +1/+1 interfaces

The discovery of two-dimensional electron gases at LaAlO3/SrTiO3 heterointerface makes it a promising candidate for new generation of nano-devices. Further enhance of interfacial transport properties is the main challenge. To increase the limit of interfacial carrier densities of two-dimensional electron gas (2DEG) of well-studied LaAlO3/SrTiO3 heterostructure from 0.5 to 1 e− per unit cell, we replaced substrate SrTiO3 by KNbO3 to acquire the +1/+1 interface with one extra electron donor and less localized valence electrons. First-principles calculations are used to study (LaBO3)m/(KNbO3)6.5 (B = Al, Ga; m = 2–8) heterostructures. The studied heterostructures all show n-type metallic state without critical thickness, and the distribution of charge densities of different monolayers can be modulated by the polarization effect of KNbO3. The interfacial carrier density of studied heterostructures perform one order of magnitude larger than LaAlO3/SrTiO3 because of the increased intrinsic carrier limit. With the increase of LaBO3 (B = Al, Ga) layer thickness, the interfacial carrier density rises up to 1.26 × 1014 cm−2, and the electron effective masses decrease monotonically, resulting in higher carrier mobility and around 6 times larger electrical conductivity. This work proved an effective approach for tuned interfacial 2DEG and future device engineering.

Ionic Bond ⇄ Covalent Bond Electron Transitions in Alloys

The ordering–separation phase transition, which has been earlier detected in many alloys and causes microstructural changes, is shown to begin with changes in the electronic structure of an alloy. In contrast to pure metals, where all valence electrons take part in the formation of a metallic bond, some pairs of valence electrons in an alloy turn out to be localized at two neighboring unlike atoms, which leads to the formation of common orbitals between them. This behavior is characteristic of the appearance of an ionic chemical bond. During the ordering–separation phase transition in an alloy, this localization of valence electrons is violated and some pairs of valence electrons become involved in the formation of hybridized orbitals between the pairs of neighboring like atoms. As a result, a covalent bond component appears instead of an ionic component in the alloy. It is concluded that the ionic bond covalent bond ⇄ electron transition precedes the microstructural ordering–separation phase transition.

Electronic structure and absorption spectra of silicon nanocrystals with a halogen (Br, Cl) coating

Ab initio calculations of the electronic structure and frequency dependence of the imaginary part of the dielectric function for 1–2 nm silicon nanocrystals with the surface fully passivated with Cl or Br halogen atoms have been performed. According to these calculations, passivation with halogens results in the strong localization of valence electrons near the surface. As a result, the width of the band gap of a nanocrystal is noticeably narrowed and its absorptance decreases as compared to the case of hydrogen passivation. These effects are more pronounced in bromine-passivated nanocrystals.

Effect of particle size on the surface activity of TiC–Ni composite coating via the interfacial valence electron localization

Electron work functions and open potentials of electrodeposited Ni, TiC micro- and nano-particle-reinforced Ni matrix coatings were studied to investigate the effects of TiC particle size on the surface activity and corrosion tendency of the coatings.

Interfacial valence electron localization and the corrosion resistance of Al-SiC nanocomposite.

Microstructural inhomogeneity generally deteriorates the corrosion resistance of materials due to the galvanic effect and interfacial issues. However, the situation may change for nanostructured materials. This article reports our studies on the corrosion behavior of SiC nanoparticle-reinforced Al6061 matrix composite. It was observed that the corrosion resistance of Al6061 increased when SiC nanoparticles were added. Overall electron work function (EWF) of the Al-SiC nanocomposite increased, along with an increase in the corrosion potential. The electron localization function of the Al-SiC nanocomposite was calculated and the results revealed that valence electrons were localized in the region of SiC-Al interface, resulting in an increase in the overall work function and thus building a higher barrier to hinder electrons in the nano-composite to participate in corrosion reactions.

ADSORPTION AND DISSOCIATION OF O2 ON Ti3Al (0001) STUDIED BY FIRST-PRINCIPLES

The adsorption and dissociation of oxygen molecule on Ti 3 Al (0001) surface have been investigated by density functional theory (DFT) with the generalized gradient approximation (GGA). All possible adsorption sites including nine vertical and fifteen parallel sites of O 2 are considered on Ti 3 Al (0001) surface. It is found that all oxygen molecules dissociate except for three vertical adsorption sites after structure optimization. This indicates that oxygen molecules prefer to dissociate on the junction site between Ti and Al atoms. Oxygen atoms coming from dissociation of oxygen molecule tend to occupy the most stable adsorption sites of the Ti 3 Al (0001) surface. The distance of O – O is related to the surface dissociation distance of Ti 3 Al (0001) surface. The valence electron localization function (ELF) and projected density of states (DOS) show that the bonds of O – O are breakaway at parallel adsorption end structures.

Electrophysical Properties of Melted Carbide ТіС, ZrC, NbC in the Range of its Homogeneity

Structure and same electrical properties of carbides of titanium, zirconium and niobium were tested in region of homogeneity. Samples of these carbides processed in the arc furnace using consumable electrodes. These samples were mono-phase, had not porosity and had minimum content of impurities of oxygen and carbon. It was found, that properties such as electrical resistivity, thermal resistance coefficient, Hall’s coefficient, concentration and mobility of charge carriers of these carbide phases vary depending on the content impurities of carbon in region of homogeneity and place of metals to create of carbides in periodic table. The nature of carbides analyses from positions of model of solid state. The base of this model is configuration of localization of valence electrons of atoms. Changing the properties of carbide phases depends on the ratio between the forces of bonds Me–C and Me–Me, which depends on the degree of stabilization sp 3 -configuration. And the degree of stabilization is depending from donor’s ability of metal which create carbide and from ratio between atoms of metal and carbon. It was established, that absolute values of properties that have been investigated similar to the values of the properties of single-crystal samples. This indicates a more ordered crystalline lattice fused refractory compounds, as well as less impurity content and absence porosity.